A fuel cell generates electricity and heat simultaneously by making a fuel gas which contains hydrogen and an oxidizing agent gas such as air which contains oxygen electrochemically react with each other. A PEFC includes: a polymer electrolyte membrane which selectively transports hydrogen ions; and an anode electrode and a cathode electrode formed on both surfaces of the polymer electrolyte membrane. These electrodes respectively include a catalyst layer formed on a surface of the polymer electrolyte membrane, and a gas diffusion layer (GDL) which is disposed outside the catalyst layer and has both air permeability and electronic conductivity. A structural body where the polymer electrolyte membrane and the electrodes are integrally assembled to each other by joining is referred to as a membrane electrode assembly (MEA).
A pair of conductive separators is arranged on both sides of the MEA. The pair of conductive separators fixes the MEA by mechanically sandwiching the MEA therebetween, and electrically connecting adjacent MEAs to each other in series. In contact portions of the separators with the MEA, a gas flow passage for supplying a reaction gas to the respective electrodes and for carrying away generated water and a surplus gas is formed. Further, to prevent a reaction gas supplied to the gas flow passage from leaking to the outside or being mixed with a gas outside the gas flow passage, a sealing member (seal) is disposed between the pair of separators such that the sealing member surrounds outer peripheries of electrode forming portions in the MEA. Such a structural body is referred to as a battery module (cell).
In a fuel cell, the enhancement of utilization efficiency of a gas is one of the important issues. As a cause of lowering of the utilization efficiency, a cross leakage of a gas, an external leakage of a gas or a shortcut of a gas is named.
From a reason such as manufacturing constraints, there may be a case where a gap is formed between an inner edge of a gasket and an outer edge of an electrode forming portion. When such a gap exists on an anode surface side, on a cathode surface side or on both the anode surface side and the cathode surface side, there is a possibility that a fuel gas and an oxidizing agent gas leak through the gap during an operation of the PEFC. The leaked fuel gas and oxidizing agent gas are discharged to the outside with being hardly exposed to the MEA. As a result, the utilization efficiency of the fuel gas and the oxidizing agent gas is lowered, which lowers efficiency (electricity generating efficiency) of the PEFC. A shortcut of a gas occurs due to such a cause.
To overcome such problems, there has been known a structure where a MEA integrated product is formed by combining a MEA, a frame body which holds the MEA and a gasket material having resiliency into an integral body (for example, see Unexamined Japanese Patent Publication No. 2009-21217).
On the other hand, a fuel cell stack has been required to satisfy downsizing and the reduction of thickness. However, a so-called carbon separator which uses carbon and a resin bonding material as base materials exhibits a large linear expansion coefficient, and hence, the carbon separator exhibits a large change in size during an operation. Accordingly, the fuel cell stack requires a spring or the like for absorbing such a change in size, and thus the fuel cell stack cannot be downsized. Further, to allow the fuel cell stack to acquire the strength, it is necessary to increase the thickness of the separator. On the other hand, there has been known a structure which enables a fuel cell stack to achieve both the reduction of a linear expansion coefficient and the reduction of thickness by using a separator where a flow passage is formed on conductive prepreg by press working (for example, see Unexamined Japanese Patent Publication No. 2006-28659).
Further, when a fuel cell is prepared by stacking a plurality of unit cells each formed by sandwiching a power generating element between a pair of separators prepared by compression molding, separators of the unit cells disposed adjacently to each other are adhered to each other with an adhesive agent. There has been known a structure where the adhesive agent functions as a seal member for sealing cooling water which flows in the separators (for example, see Unexamined Japanese Patent Publication No. 2005-166425).